FIG. 5 is a circuit diagram illustrating an example of a configuration of the main parts of a conventional overcurrent protection circuit for use in a vehicle.
This overcurrent protection circuit is a circuit that protects a switching element 7 connected between the plus electrode of a battery 11 and the supply terminal of the load 10. The ground terminal of the load 10 is grounded.
The switching element 7 has a configuration in which the source of an N-channel MOSFET (metal-oxide semiconductor field-effect transistor) 8 whose drain is connected to the plus electrode of the battery 11, and the source of an N-channel MOSFET 9 whose drain is connected to the load are adjacent and connected to each other. The FETs 8 and 9 are drive-controlled by a driver (driving circuit) 5 so as to be turned on or off at the same time.
Furthermore, one end of a resistor 4 is connected to the plus electrode of the battery 11, and the other end of the resistor 4 is connected to the input terminal of a constant current circuit 3 and the non-inverting input terminal of a comparator 2. The output terminal of the constant current circuit 3 is grounded, and the inverting input terminal of the comparator 2 is connected to the supply terminal of the load 10 via a resistor 6.
The output terminal of the comparator 2 is connected to one input terminal of an AND circuit 1, an inverted signal of a mask signal is input to the other input terminal of the AND circuit 1, and an output of the AND circuit 1 is given to the driver (driving circuit) 5. The mask signal, and a command signal for the driver are given from a not-shown ECU (Electronic Control Unit) of the load 10.
Hereinafter, examples of operations of an overcurrent protection circuit having such a configuration will be described with reference to the timing diagrams of FIGS. 6 and 7 showing that example.
Note that the output voltage value of the battery 11 is assumed to be constant (FIGS. 6A and 7A).
The constant current circuit 3 generates a threshold voltage of a drop voltage by letting a constant current flow through the resistor 4. When an overcurrent flows through the switching element 7, and the value of a drop voltage caused by the on-resistance of the switching element 7 increases to a value larger than the value of the drop voltage generated by the constant current of the resistor 4 (the voltage value itself decreases), the comparator 2 needs only to output a plus signal so as to stop the driver 5.
However, when the switching element 7 is off (FIGS. 6B and 7B), a differential voltage value between both ends of the switching element 7 (a switching element potential difference of FIGS. 6C and 7C) is at the level of the voltage value of the battery 11 and exceeds the threshold voltage. Therefore, a mask time period (FIGS. 6E and 7E) in which output of the comparator 2 is disabled needs to be provided for a time period in which the switching element 7 is off, and a time period in which the state of the switching element 7 is transient from off to on.
In this overcurrent protection circuit, the mask time period is configured such that an inverted signal of a mask signal is given to the other input terminal of the AND circuit 1, and thus, during the mask time period, the AND circuit 1 continues to output an L-level signal and does not stop the driver 5 even when the output signal of the comparator 2 becomes positive.
Accordingly, in this overcurrent protection circuit, when the output of the driver 5 is off (FIGS. 6B and 7B), a mask signal is applied to realize the mask time period (FIGS. 6E and 7E), and thus the output signal of the comparator 2 is disabled although the differential voltage value between both ends of the switching element 7 (switching element potential difference of FIGS. 6C and 7C) exceeds the threshold voltage (overcurrent detection threshold).
Also in the transient time period in which the output of the driver 5 changes from off to on (FIG. 6B), a mask signal is applied to realize the mask time period (FIG. 6E), and thus the output signal of the comparator 2 is disabled although the differential voltage value between both ends of the switching element 7 (switching element potential difference of FIG. 6C) exceeds the threshold voltage (overcurrent detection threshold).
A time period in which the output of the driver 5 steadily remains on (FIG. 6B) serves as a non-mask time period since no mask signal is applied (FIG. 6E).
In this state, if a ground fault occurs in the load 10 on the supply terminal side (FIG. 6D), the current flowing through the switching element 7 will increase (FIG. 6D) and the differential voltage value between both ends of the switching element 7 (FIG. 6C) will exceed the threshold voltage (overcurrent detection threshold). Accordingly, the comparator 2 outputs a plus signal, the AND circuit 1 outputs an H-level overcurrent detection signal, the output signal of the driver 5 changes from on to off (FIG. 6B), and the current flowing through the switching element 7 starts decreasing before reaching a breakdown current and eventually takes on zero (FIG. 6D).
Patent Document 1 discloses a load circuit overcurrent protection device that is capable of highly accurate overcurrent detection without being affected by an on-resistance deviation ±ΔRon of a semiconductor element.